Definitions for knee-jerk
Random House Webster's College Dictionary
a reflex extension of the leg resulting from a sharp tap on the patellar tendon.
Origin of knee jerk:
Informal. reacting in an automatic, habitual manner; unthinking:
a knee-jerk liberal.
Category: Common Vocabulary
Origin of knee-jerk:
A sudden reflexive movement of the leg below the knee, as a reaction to a tap to the tendon just below the patella (kneecap).
Automatic, spontaneous, easily predictable
He gave a knee-jerk response.